1. Field of the Invention
The present invention generally relates to the field of machines for injection molding hollow plastic articles, and more particularly concerns machines for injection molding hollow plastic articles which are subsequently reheated and blow molded into containers such as oriented hollow plastic containers.
2. Description of the Prior Art
Hollow plastic articles or preforms produced by injection molding generally have relatively thick walls and are molded at relatively high temperatures. After removal from the mold, the hollow plastic articles must be cooled sufficiently to prevent their deformation or adhesion to one another. However, their thick walls act as an insulator tending to trap heat inside the wall. A high rate of production is important in commercial operations, and the rate at which the injection molding cycle can produce preforms is limited by the time taken to cool the preforms sufficiently to allow post molding handling without damage to the preform. Molding cycles have been reduced to the point that there is insufficient time left for the molded articles to be completely cooled in a transporter before it is needed to retrieve the molded articles from the next molding cycle.
Cooling the articles in the mold by means of thermal contact with cooled mold surfaces quickly chills the inner and outer skins, but mold cooling removes the heat trapped within the wall only by keeping the articles in the mold for an extended period of time, which is economically undesirable. Initially cooling the skin of the molded articles may be sufficient to permit ejection from the mold without damage, but additional cooling is then immediately required to remove additional heat conducted to the skin. If the additional cooling were omitted, the skin temperature would rise and tend to cause the molded articles to stick together, to become prone to surface damage, to bend, or warp, and or to crystallize.
One plastic article molding machine capable of rapid cycling is disclosed in U.S. Pat. No. 4,721,452 and includes a carrier plate cooperating with an injection molding machine and having a plurality of sets of article receivers for cooling preforms, with the number of article receivers corresponding to a multiple of the number of preforms produced in an injection molding cycle. The molding machine further includes means for positioning one set at a time of the carrier plate cavities in juxtaposition with formed preforms, and means for transferring the preforms to selected article receivers. Unfortunately, during operation of the molding machine, the carrier plate tends to move out of properly aligned position within the gap between the first and second mold portions, with the preforms thereby subjected to undesired and perhaps damaging contact with the molding machine. Similar problems can also occur with other article receivers used in conjunction with molding machines such as those disclosed in U.S. Pat. Nos. 4,140,464; 4,690,633; 4,814,134; 4,915,611; 5,206,039; 5,338,172; 5,354,194; 5,443,360 and European Patent Publication EP 0 718 084 A2.